1. Field of the Invention
The present invention relates to a heat-sealing apparatus for use in a filling/packing machine for producing liquid beverages or the like which are packed in paper containers so that they can be stored for a long time and, more particularly, to a sealing apparatus for transversely heat-sealing a tubular packing material filled with contents, such as liquid beverages, together with fluid.
2. Description of the Related Art
Conventionally, when producing a packing container having a box shape filled with a liquid such as juice that is to be sealed, as shown in Japanese Patent Publication No. 3215/1980, it is well known to use a sealing apparatus in which a packing material including a laminate of paper, aluminum foil and a synthetic resin is continuously formed into a tubular shape while being conveyed. As shown in FIG. 1, the tubular packing material 1 is transversely heat-sealed with fluid by using pressing means including a sealing jaw 3 having a high-frequency coil 2 and an opposing jaw 5 having a sealing rubber 4 of hard rubber. Tubular material 1 is cut between two sealed zones having been heat-sealed by a cutting blade 6.
In Publication of Japanese Patent application Laid-Open Nos. 134744/1983, 269854/1993, 164523/1995 and 218805/1996, there is further disclosed a heat-sealing apparatus for heat-sealing a laminate material in which a ridge is formed on the pressing face of a high-frequency coil on a sealing jaw in the aforementioned heat-sealing apparatus so that molten resin is extruded into a sealed zone by the ridge.
In known heat-sealing apparatus, when molten resin is guided to flow toward a container's interior side of the sealed portion of a tubular packing material, corrugated molten resin beads are formed on the edge portion of the container's interior side so that cracks start from the crests of the beads to break the container when an external force is applied to the container. In order to avoid the above problem of the aforementioned heat-sealing apparatus, two magnetic members are arranged on the outer side (or the container's interior side) of the straight portions of a U-shaped high-frequency coil to leave a portion of the container's inner side unheated, as disclosed in Japanese Patent No. 2571977.
In the Publication of Japanese Patent Application Laid-Open No. 230834/1996, a high-frequency coil having a ridge in its longitudinal direction and grooves formed on the two sides of and in parallel with the ridge causes melted resin to remain in the grooves. Thus, the melted resin does not flow out of a sealed zone, even if the resin on the packing material is melted and pushed by the ridge. It is further described in heat-sealing apparatus of the prior art which do not have an aforementioned groove that the molten resin having flowed out of a sealed zone solidifies to stick to the interior side of a packing container thereby forming cracks during a shaping process after the sealing step.
In the Publication of Japanese Patent Application Laid-Open No. 244728/1996, on the other hand, a high-frequency coil which is inclined gradually at a greater distance than the sealed zone forming portion goes to the outer side (to the container's interior side) is employed so that the tube is heat-sealed whereby liquid and molten resin are discharged smoothly from the sealed zone to the container inner side, when the tube is pressed together with fluid.
In order to effect an excellent heat-sealing in a sealing apparatus for transversely heat-sealing a tubular packing material filled with contents such as a liquid beverage in the presence of the liquid, it is necessary to exclude the liquid from the sealed zone of the tube as much as possible when the tube is sealed with heat and under pressure. However, the inner face of a tube which is in contact with the liquid is not completely flat and has small irregularities, if observed in detail. In the heat-sealing apparatus disclosed in the aforementioned Publication of Japanese Patent Application Laid-Open Nos. 134744/1983, 269854/1993, 164523/1995, 240607/1997 or 230834/1996, a sealing method is adopted in which the molten resin can be prevented from flowing out of the sealed zone so that it remains in the sealed zone. Therefore, any liquid or dirt which may have entered the small irregularities in the tube inner face cannot be discharged to the outside of the sealed zone so that the sealing property cannot be said to be sufficient.
On the other hand, the inventors of this invention have discovered that any liquid in the sealed zone may be guided to flow out together with the molten resin to the outside of the sealed zone so as to clear the liquid or dirt in the irregularities when sealed, and have competed the invention, as disclosed in the aforementioned Publication of Japanese Patent Application Laid-Open No. 8-244728. The method using this heat-sealing apparatus can clear the liquid or dirt in the irregularities completely to provide an excellent sealing property, but the molten resin having flowed out to the container's interior side may not be homogeneously extruded. As shown in FIG. 2, undulating molten resin beads 7 are formed on the edge portion of the container's interior side. During a secondary step using a shaping machine after the sealing step, it has been found out that cracks start from crests 8 of the undulations of the beads 7 to cause liquid leakage although not frequently when a pressure is applied to the container.